Powered airborne craft, manned and unmanned, may be capable of hovering in a stationary position while airborne. Such aircraft may range from craft which operate close to the ground relying on a cushion of air to those capable of free flight and vertical takeoff and landing. Craft operating close to the ground may be designed for transportation and recreational use whereas the free flight craft may operate at generally low altitudes compared to commercial aircraft and may be considered for applications including airport-to-downtown shuttle, tourism applications, home-to-office commuting, search and rescue and surveillance operations.
In a craft free of ground effect, lift can be generated by the acceleration of a mass of air by a fan, propeller, wing, or other system. When a mass of air is changed from rest to a given velocity in a downward direction, an upwardly directed reaction force is produced. In general, the more air that is directed, the less power is required to produce a given lift. This defines a technical challenge because increasing the volume of air generally involves an increase in the size of the craft as evidenced in the large diameter, high speed blades used in helicopters.
To address the above challenge, the applicant has developed an aerodynamic lifting device for airborne craft that provides a more compact form of craft, than a helicopter, and which uses fan blades which are more evenly loaded than comparable helicopter blades. The device uses a drum fan type rotor in an airborne craft with a relatively small footprint and, typically, the drum fan type rotor will be lightweight construction to minimize weight and power required to lift the device. The fan may be described as a drum rotor or radial drum fan, that is, a fan with the blades advantageously occupying an annular region having a radial depth that is less than 25% of the radial pitch of the blades. By placing the rotor blades at a distance from the rotational axis of the fan, a central region within the rotor is conveniently provided for a payload, or in the case of a larger sized craft, a pilot and/or passengers.
The use of such a drum rotor type fan also provides other benefits. One such benefit is that effectively the entire length of a blade is being fully utilized as an aerodynamic device (as compared to the tip of the helicopter blade, described above) since it is vertically disposed and the airflow is radial. Additionally, the design of the drum rotor allows for each blade to be supported at either end via upper and lower support rings (again, as opposed to the cantilevered design of the helicopter blade). Also, simple constant cross-section blade profiles may be used which offer manufacturing cost savings (as opposed to helicopter blades which utilize a complex lengthwise twist to provide the proper angle of attack along the length of the blade).
The rotor must be driven to rotate through a torque transmission means for transmitting torque from a prime mover, and typically, as will become apparent from the discussion below, a plurality of prime movers arranged about the periphery of the rotor body. Torque transmission to drive the rotor of the aerodynamic lifting device raises a number of challenges.
First, the torque transmission means must be capable of transferring the very high levels of tractive effort needed to develop the required high levels of power in the rotor as required to generate lift.
Second, the torque transmission means should be capable of damping any instantaneous high forces to avoid damage to the rotor structure.
Third, radial or normal loads required to transmit the required tractive effort within the torque transmission means should be held to a minimum to avoid damage to the rotor structure and to minimize weight of the craft support structure or chassis.
Fourth, the torque transmission means must be capable of restraining any loads generated by the rotor. In particular, the drive means must be capable of restraining the gyroscopic loads generated both upward and downward in the direction of the axis of the rotor when the rotor rolls or pitches during manoeuvres.
The Applicant has tried a number of design approaches for the torque transmission means. As shown in FIG. 1, the torque transmission means comprises a plurality of prime movers—in the form of internal combustion (IC) engines 110 and associated drive systems—arranged at 120 degree intervals arranged to extend outward of the periphery 11 of the rotor 112 and supporting triangular frame 177. The periphery is formed by a circumference 114 of a drive rim 113 forming the lower portion of the rotor 112. The engines 110 each drive a tooth belt 175 with a flat back 122 providing a friction contact with the rotor 176 and the tooth side of the belt 175 being driven by motor pulleys 186 connected to each of the engines 110 and transmitting torque to the rotor drive rim 118 of the rotor 112. Vertical and radial restraint of the rotor 112 is provided by additional rollers 187 acting on both the radial outer face and axial upper face of the rotor drive rim 118.
This roller arrangement is complex to compensate for gyroscopic forces. In addition, extremely high instantaneous and local loads are directly transmitted to the belt and rotor as well as the rollers. Premature failure is therefore a risk and, in any event, undesirable weight and complexity is added to the device.